1. Field of the Invention
The present invention relates to a shift register, and more particularly, to a shift register capable of compensating a threshold voltage variation of a transistor.
2. Description of the Related Art
With a rapid development of monitor types, novel and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDAs), digital cameras, and projectors. The demand for the novelty and colorful monitors has increased tremendously.
Referring to FIG. 1 showing a functional block diagram of a conventional liquid crystal display 10, the liquid crystal display 10 includes a liquid crystal panel 12, a gate driver 14, and a source driver 16. The liquid crystal panel 12 includes a plurality of pixels, each pixel having three pixel units 20 indicating three primary colors of red, green, and blue. For example, the liquid crystal display 12 with 1024 by 768 pixels contains a number of 1024×768×3 pixel units 20. The gate driver 14 periodically outputs a scanning signal to turn on each transistor 22 of the pixel units 20 row by row, meanwhile, each pixel units 20 is charged to a corresponding voltage based on a data signal from the source driver 16, to show various gray levels. After a row of pixel units is finished to be charged, the gate driver 14 stops outputting the scanning signal to this row, and then outputs the scanning signal to turn on the transistors 22 of the pixel units of the next row. Sequentially, until all pixel units 20 of the liquid crystal panel 12 are finished charging, and the gate driver 14 outputs the scanning signal to the first row again and repeats the above-mentioned mechanism.
As to the conventional liquid crystal display, the gate driver 14 functions as a shift register. In other words, the gate driver 16 outputs a scanning signal to the liquid crystal display 12 at a fixed interval. For instance, a liquid crystal display 12 with 1024×768 pixels and its operating frequency with 60 Hz is provided, the display interval of each frame is about 16.67 ms (i.e., 1/60 second), such that an interval between two scanning signals being applied on two row adjacent lines is about 21.7 μs (i.e., 16.67 ms/768). The pixel units 20 are charged and discharged by data voltage from the source driver 16 to show corresponding gray levels in the time period of 21.7 μs accordingly.
Output of the transistor inside the gate driver 14 is pulled up because of the capacitance coupling effect between a drain and a gate when a clock signal is at high voltage level. This would probably lead to the transistor T2 being turned on unintentionally. In this way, each stage of the shift register outputs unexpected pulse, causing the panel to display abnormally. As for the gate driver 14 that is utilized in high resolution LCD panels, which are manufactured with amorphous silicon thin film processing technology, in particular, evenness and stability of components in the processes have great variability. So, if the transistor inside the gate driver 14 is turned on unintentionally after the LCD panel 12 is lightened, the damage to the LCD panel 12 would be extraordinarily serious.